The present invention relates to a magnesium oxide target for use in forming a magnesium oxide layer for magnetic recording mediums of magnetic disk devices or tunneling magnetoresistance (TMR) elements and other electronic devices, and to the method of producing such a magnesium oxide target; and particularly relates to a sintered compact magnesium oxide target for sputtering of high purity and high density and which is free of color shading that occurs at the center of the target, and to the method of producing such a sintered compact magnesium oxide target for sputtering.
In recent years, pursuant to the downsizing and higher recording density of magnetic disks, research and development of magnetic recording mediums are being conducted, and in particular Co-based magnetic layers and under-layers have been improved variously. Meanwhile, the recording density of hard disks has been increasing rapidly year by year, and it is considered that current surface density of 600 Gbit/int will reach 1 Tbit/in2 in the future. When the recording density reaches 1 Tbit/in2, the recording bit size will be less than 10 nm and, in such a case, it is anticipated that the superparamagnetism caused by thermal fluctuation will become a problem. The currently used magnetic recording medium structure such as a structure with increased magnetic crystalline anisotropy obtained by adding Pt to a Co—Cr-based alloy will become insufficient. This is because magnetic particles that behave with stable ferromagnetism at a size of 10 nm or less require even greater magnetic crystalline anisotropy.
Due to the foregoing reason, a Fe—Pt phase having an L10 structure is attracting attention as a structure for use in an ultra-high density recording medium. Since the Fe—Pt phase having an L10 structure possesses high magnetic crystalline anisotropy in addition to yielding superior corrosion resistance and oxidation resistance, it is expected to be a structure that can be suitably applied to a magnetic recording medium.
When using a Fe—Pt layer as a structure for use in an ultra-high density recording medium, it is demanded to develop a technology of dispersing ordered Fe—Pt magnetic particles regulated in the same direction with a density as high as possible in a magnetically isolated state. While it is necessary to control the crystal orientation to provide magnetic anisotropy to the Fe—Pt thin film, this can be easily performed by selecting a single crystal substrate. In order to vertically align an easy axis, it has been reported that a magnesium oxide film is suitable as the under-layer of the Fe—Pt layer.
In addition, it is also known that a magnesium oxide film can be suitably used as the insulating layer, i.e., tunnel barrier of a TMR element that is used in a magnetic head (for hard disks) or an MRAM. While the foregoing magnesium oxide film has been conventionally formed via the vacuum deposition method, in recent years the sputtering method is being used to produce magnesium oxide films from the perspective of simplification of the production process and facilitation of the production of large screens.
There are the following publications as conventional technology.
JP-A-H10-130827 describes a magnesium oxide target made from a magnesium oxide sintered compact having a magnesium oxide purity of 99.9% or higher than a relative density of 99% or higher, wherein the magnesium oxide target has a fine structure in which the average grain size is 60 μm or less and round pores having an average grain size of 2 μm or less exist in the crystal grains, and is compatible up to a sputter deposition rate of 1000 Å/min or more. This technique is based on a method of adding fine magnesium oxide powder having an average grain size of 100 nm or less to high purity magnesium oxide powder and mixing and compacting the powders and subjecting the obtained compact to primary sintering and secondary sintering.
JP-A-H10-130828 relates to a magnesium oxide target made from a magnesium oxide sintered compact having a relative density of 99% or higher and capable of achieving a deposition rate of 500 Å/min or higher in sputter deposition performed in an Ar atmosphere or Ar—O2 mixed atmosphere, and proposes compacting high purity magnesium oxide powder having an average grain size of 0.1 to 2 μm based on CIP at a pressure of 3 t/cm2 or higher, and sintering the obtained compact.
JP-A-H10-158826 describes a magnesium oxide target made from a magnesium oxide sintered compact having a magnesium oxide purity of 99.9% or higher and a relative density of 99.0% or higher, and compatible up to a sputter deposition rate of 600 Å/min or more. This technique is based on a method of adding electromelted magnesium oxide powder and fine magnesium oxide powder having an average grain size of 100 nm or less to high purity magnesium oxide powder and mixing and compacting the powders and subjecting the obtained compact to primary sintering and secondary sintering. JP-A-H10-158826 describes that a magnesium oxide film having favorable orientation, crystallinity and film properties can be deposited via the sputtering method at a high deposition rate.
JP-A-H10-237636 describes a target having MgO as its main component, as well as a method for producing such a target, and proposes dispersing La particles, Y particles and Sc particles in a target having MgO as its main component for use as a protective film of a dielectric layer of an Ac-type PDP in order to achieve a low discharge voltage, sputtering resistance during discharge, quick responsiveness to discharge, and insulation properties.
JP-A-H11-6058 proposes, in a target having MgO as its main component, dispersing LaB6 particles in the MgO matrix, performing reduction treatment in a reduced gas atmosphere prior to sintering, and performing primary sintering and secondary sintering at a predetermined temperature in order to improve the strength, fracture toughness value, and resistance to thermal shock.
JP-A-H11-335824 prescribes the relative density and the average crystal grain size to be 0.5 to 100 μm in a target having MgO as its main component, and dispersing the rare earth elements of Sc, Y, La, Ce, Gd, Yb, and Nd in the MgO matrix.
JP-A-H11-139862 proposes sintering a MgO green compact based on the spark plasma sintering method in order to produce a high density sintered compact.
JP-A-2009-173502 and WO2009/096384 describe methods of obtaining a MgO sintered compact with numerous (111) planes aligned based on uniaxial pressure sintering and having an ultimate density of 3.568 g/cm3 so as to achieve favorable mechanical property and thermal conductivity, and reduction in contamination of the atmosphere caused by the generation of gas, and propose subjecting MgO raw material powder having a grain size of 1 μm or less to uniaxial pressure sintering, and subsequently performing heat treatment in an oxygen atmosphere at a temperature of 1273 K or higher. In the foregoing case, MgO is used as the raw material powder, and the method of increasing the density is limited to the sintering conditions.
JP-A-2000-169956 proposes a target for depositing a MgO film in a large size and uniform manner. In addition to prescribing the average crystal grain size, the density, the deflective strength, and the center line average roughness of the target surface, JP-A-2000-169956 proposes causing the grain size of the raw material powder to be 1 μm or less, subjecting the raw material powder to granulation, sintering the granulated raw material powder at a predetermined load and temperature, and finishing the surface of the target to achieve a center line average roughness Ra of 1 μm or less. Incidentally, while not directly related to the present invention, aforementioned JP-A-H10-130827, JP-A-H10-130828, JP-A-H10-158826, JP-A-H10-237636, JP-A-H11-6058, JP-A-H11-335824, JP-A-2009-173502, and WO2009/096384 describe the evaluation of the “bending strength” of a target, and JP-A-2000-169956 describes the evaluation of the “deflective strength” of a target.